Numerous portable electronic devices such as laptops, cell phones, digital cameras, electric shavers or the like are not “truly portable” as long as they have to be routinely charged by the line power. A wireless power transmission technique capable of delivering electromagnetic energy to these portable devices without human intervention would make them truly tether free. The relationship between wireless charging and wired charging resembles that between wireless Internet and wired Internet. A “wireless charger” and a wireless router share similar functionality: they both construct wireless links to portable devices. However, they differ in that a wireless router transfers data, whereas a wireless charger broadcasts power.
Wireless charging may be omni-present in the near future as wireless Internet is nowadays, with applications of wireless power transmission seemingly unbounded. As described above, wireless charging may make numerous electronic devices “truly portable” or “truly tether free.” Wireless power delivery may be especially valuable in scenarios where wired connections are intractable. For example, if unattended radio frequency identification tags and implanted sensors are powered remotely, they would be free of battery life restrictions and in turn significant functionality enhancements are expected. Furthermore, when applied in conjunction with renewable energy sources (such as wind and solar), wireless power transfer may enable fundamentally new energy scavenging systems with high efficiency and low cost.
Many have investigated power transmission using radio frequency (RF) waves for over a century, the first arguably being Tesla in 1893. However, major advances in this area have been made in the past decade. The most well-known application might be using microwave beam to deliver power from spacecrafts to the earth, although its feasibility is still under evaluation. A case study from 1997 to 2004 constructed a point-to-point wireless electricity transmission to a small isolated village called Grand-Bassin in France. In 2007, a team of researchers from Massachusetts Institute of Technology demonstrated the ability to power a 60-Watt light bulb over two meters using an inductive resonance coupling scheme that makes use of near-field coupling between two magnetic resonators. In 2008, the University of Colorado developed an antenna array to harvest 100 mW of power from an RF transmitter one meter away. In 2009, the Journal of Sound and Vibration published a study of the feasibility of using a car-borne power broadcaster to power sensors installed over a bridge. In addition to the above research endeavors, several companies have developed products targeting for specific applications. For instance, cordless toothbrushes and wirelessly chargeable laptop computers are commercially available. As another interesting application, Alticor's eSpring™ water is processed using wirelessly powered ultraviolet lamps.
Notwithstanding the above and other similar investigations, a large gap still exists between existing technologies and a practical, general-purpose and ubiquitous wireless power transmission system. Specifically, in order to reliably charge portable devices in complex environments of everyday life, several technical challenges must be addressed, including efficiency improvement, safety assurance, and cost, size and weight reduction.
During wireless power transmission, power loss is due to many factors, most notably RF-to-direct-current (RF-DC) conversion and RF propagation. Recent development of rectifying antennas (rectennas) has significantly mitigated RF-DC conversion loss, and spatial beamforming (that is, spatial focusing of electromagnetic radiation) may be an effective means for improving the RF propagation efficiency. Beamforming may be relatively simple for stationary devices with high-gain/highly-directive antennas, but it remains challenging for multiple mobile/portable devices residing in a large area. Traditional phased-array beamforming may not be an ideal solution, as it may fail when the line-of-sight path between the phased-array and the target device is obstructed by obstacles.
As radiation of high-frequency radio waves is potentially harmful to human beings, it may also be challenging to deliver sufficient power to portable devices while ensuring human safety.
Further, some existing solutions may be unsuitable for ubiquitous deployments due to high cost, large size and/or heavy weight. One existing solution, for example, uses three-dimensional coils with radius 25 cm for wireless power reception.
Therefore, it would be desirable to have a system and method that takes into account at least some of the issues discussed above, as well as possibly other issues. More particularly, for example, it would be desirable to have a system for wireless power transmission that includes planar, smaller and lighter apparatuses, and that demonstrates high power efficiency and little hazardous impact.